Electrophotographic imaging and developing processes have been extensively described in the patent literature. As described in U.S. Pat. No. 2,297,691 to Carlson, the basic process involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas exposed to the light and developing the resulting latent electrostatic image by depositing on the image a finely divided electroscopic material known as "toner". The toner may normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the latent electrostatic image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat.
Various methods for applying the toner particles to the latent electrostatic image are known. In the "cascade" method, as described in U.S. Pat. No. 2,618,552 to Wise, toner particles charged triboelectically to the desired polarity are conveyed to and rolled or cascaded across the electrostatic latent-image-bearing surface. The toner particles are electrostatically deposited on and secured to the charged portion of the latent image. Most of those toner particles accidentally deposited in the background are removed by the rolling carrier due, apparently, to a greater electrostatic attraction between the toner and the rolling carrier than between the toner and the discharged background.
In another method, the "magnetic brush" method, disclosed, for example, in U.S. Pat. No. 2,874,063, developer material containing toner and magnetic carrier particles are carried by a magnet. The magnet's field causes the magnetic carrier to align in a brush-like configuration. When the "magnetic brush" is engaged with the electrostatic latent-image-bearing surface, the toner particles are drawn from the brush to the latent image by electrostatic attraction.
Other patents describing the electrophotographic process, and in particular various toner compositions, include U.S. Pat. Nos. 3,609,082 to Moriconi et al., 4,508,806 to Oseto et al., 4,859,558 to Matsumura et al. and 3,893,935 to Jadwin et al. As these patents indicate, typically the toner composition comprises a resinous binder suitably colored or darkened, for contrast purposes, with a colorant like dyestuffs or pigments such as carbon black. Toner particles may be used as single component "developers" or alternatively, as described in U.S. Pat. No. 3,893,935 to Jadwin et al., they may be combined with a carrier vehicle that can either be a magnetic material such as iron filings, powdered iron or iron oxide, or a triboelectrically chargeable non-magnetic substance like glass beads or crystals of inorganic salts such as sodium or potassium fluoride.
In addition to the resinous binder and colorant components of the toner composition, a variety of additives for modifying the surface properties of particulate toner particles have been described. For example, various charge control agents may be used to modify the surface properties of the toner powder so that a uniform, stable high net electrical charge may be imparted to the toner powder by the particulate carrier vehicle. Examples of charge control agents include the mono- or di-functional organic and nigrosine salts disclosed in U.S. Pat. No. 3,647,696 to Olson, the long chain quarternary ammonium surfacants disclosed in British Patent No. 1,174,573 and the quarternary ammonium salts described in U.S. Pat. No. 3,893,935 to Jadwin et al.
In addition to charge control, two other important properties of a toner composition are low temperature fusibility and pulverizability. By pulverizability we mean the relative ease with which the toner composition may be ground to very small particles. Relatively low temperature fusibility is important because the toner must be readily fusible without excessive energy cost and without scorching or charring of the paper support. Unfortunately, conventional toner materials that are easily fused by heating sometimes tend to cake or agglomerate during handling and storage and tend to form tacky images. This agglomeration or "blocking" phenomenon, as it is known, is described in, for example, U.S. Pat. Nos. 4,508,806 to Azar et al. and 3,980,575 to De Roo et al. Agglomeration or blocking is directly at odds with the high resolution of electrophotographic images which depends in part on the presence of small discrete particles of toner to define only the latent image area without edge overlap, smear, etc.
For this reason, liquid toners have been favored for electrostatographic applications directed to optimum image resolution because the small size of the toner particles allows finer edge discrimination. Unfortunately, stable multiphase dispersions of solid particles in a liquid medium are difficult to prepare, to store, and to use. Preparation of such toners involves delicate balancing of surfactants which provide dispersion stability, with charge control agents which control image discrimination of the particle for the latent image site. The dispersions employ large quantities of flammable organic solvents as the dispersion media, and these solvents must be removed from toned prints. Further, being flammable, malodorous organic solvents, measures must be taken to prevent environmental contamination during use.
One attempt to solve the apparent incompatibility of these two desirable properties, i.e., low temperature fusibility and good pulverizability is described in U.S. Pat. No. 4,233,388 to Bergen et al. As described therein, one disadvantage of conventional easily fusible toner compositions, which typically employ low molecular weight resins as the binder, is that they tend to form thick films on reusable photoconductor surfaces. These films tend to cause image degradation and contribute to machine maintenance down-time. Similarly, low molecular weight resin toner compositions tend to "impact" or become welded on the surface of carrier particles, thus adversely affecting the triboelectric properties of developer mixtures. Furthermore, some low molecular weight resins are difficult or even impossible to pulverize in a conventional grinding apparatus.
In U.S. Pat. No. 4,233,388, Bergen et al. proposed as a solution to these problems a method for making toner particles in which low molecular weight polymers are first melted and blended with other toner components such as colorants. Films or fibers of the colored polymer materials are then preferably subjected to a molecular orientation step to improve their mechanical properties, and are finally embossed or cut to the desired particle size by passing the films or fibers through embossing or cutting rollers.
By this method, low molecular weight polymers are made as resistant to film formation and impaction as conventional resin polymers, and toner particles of any size within the range from about 1 to about 30 microns may be obtained. This complex process, however, results in additional steps in the preparation of toner compositions, and therefore, additional expense.
There is, therefore, a need for solid electrographic toner compositions which can be readily ground to unusually fine particle size (i.e., less than about 5.mu.) to improve the resolution of electrographic images toned with said solid toner developers, while being fusible at relatively low temperatures.